charging the bootstrap cap

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Lots of half-bridge and full-bridge mosfet gate drivers use a
bootstrap capacitor to make the necessary gate drive for the upper
n-fet. The upper-fet gate needs to swing maybe 10 volts above the
drain supply, which is generally the biggest voltage around.

But the first time the driver is enabled, the boot cap may not be
charged. And it will eventually discharge, given a long enough
positive output pulse. I've never seen a data sheet mention this.

I think I recall some driver that has a charge pump, but that's rare.

Halfway driving a fet is a good way to blow it up. I haven't done that
this week, but it is a concern.




Re: charging the bootstrap cap
onsdag den 13. januar 2021 kl. 21.52.36 UTC+1 skrev John Larkin:
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some have UVLO on both high and low side voltage


Re: charging the bootstrap cap
On Wed, 13 Jan 2021 13:16:24 -0800 (PST), Lasse Langwadt Christensen

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I don't think that helps the bootstrap problem.

It does help to pull the output node down to ground with some
resistors or something... in some cases.


Re: charging the bootstrap cap
onsdag den 13. januar 2021 kl. 23.18.20 UTC+1 skrev John Larkin:
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it stops it from trying to turn on the high side when it can't and forces you  
to do the right things which is, charge the bootstrap cap before you start  
switching and keep the duty cycle in a range that keeps the bootstrap cap charged


Re: charging the bootstrap cap
13.01.21 23:18, John Larkin  wrote:
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UVLO for both high and low side is essential for robust design

About the bootstrap, you always turn the low side on at startup to charge the cap. Some even apply small duty cycle to limit the inrush current

Common for bootstrapped half bridge is that 100% duty cycle is not allowed, at least not for many longer periods  

--
Klaus Kragelund

Re: charging the bootstrap cap
John Larkin wrote:


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You really want the low-side transistor on when possible to keep the  
bootstrap cap charged.  And, then, the low-side needs to be turned on some  
of the time, when the PWM is off, to keep it charged.

Jon

Re: charging the bootstrap cap
wrote:

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One resistor is a lot easier than adding more logic.



--  

John Larkin      Highland Technology, Inc

The best designs are necessarily accidental.


  

Re: charging the bootstrap cap
John Larkin wrote:

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Right, but this approach is not intended for "long enough output  
pulses". Use an SN6501 and a tiny trafo wound on a 6-10mm toroidal core  
and there will be no low frequency limit.

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Isn't it kind of obvious?

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No high-performance driver I know of has this feature. SN6501 is dirt  
cheap and can power more than one driver.

    Best regards, Piotr

Re: charging the bootstrap cap
On Thu, 14 Jan 2021 09:53:58 +0100, Piotr Wyderski

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If the controller is counting on the upper driver to perform in  
response to a falling charge pump voltage in a predictable way,  
adding an external source could open a new can of worms.  

I don't see an inhibit line . . . .

RL

Re: charging the bootstrap cap
Am 13.01.21 um 21:52 schrieb John Larkin:
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I had good results with these when biasing a GaASFET:

<  
https://www.digikey.de/product-detail/de/vishay-semiconductor-opto-division/VOM1271T/VOM1271TTR-ND/3588700
    >

A LED and some photo cells. Does not create any noise.

Gehard

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